This invention relates in general to engine control systems, and more particularly, to a method and system for correcting static error in engine speed measurements. Static errors are fluctuations in engine speed that result from mechanical part and installation tolerances, etc., and are not directly related to engine imbalances due to fuel injections.
In a multi-cylinder reciprocating internal combustion engine, there are differences in the amount of useful torque produced by each cylinder, even during normal operation. Small between-cylinder torque differences can cause rough idling and poor emissions performance. Large between-cylinder torque differences can cause extremely rough operation and may indicate faulty cylinder and/or fuel injection components. Many inventions have been devoted to sensing these torque differences and using the information for compensation or diagnosis of engine operation.
Non-uniform cylinder torque can be observed as small engine speed fluctuations about the average engine speed at any given operating point of the engine. There are many engine systems that utilize the observed engine speed fluctuation in an engine cycle to balance the power output of the cylinders. Most present day electronically controlled engines include some form of interrupt-based engine speed sensing mechanism, which allows measurement of engine speed at very short intervals. However, the accuracy of engine speed measurement is always limited by manufacturing tolerances in the involved engine parts. These irregularities include, for example, tooth-to-tooth spacing errors in cam or crankshaft gears, and gear installation stack-up errors, and other engine components and assembly tolerances, etc. The combined effect of these irregularities is the so-called static errors in measured engine speed. The static errors can sometimes be significant; but due to the prohibitive cost of machining and installing precision gears, these errors are commonly ignored and assumed to be constant from shot to shot. Therefore, the effect of balancing cylinder power output based on the uncompensated engine speed data can be seriously compromised.
Various prior art inventions seek to compensate for the static errors in measured engine speed data. U.S. Pat. No. 5,696,316 to Mezger, et al., discloses a system for compensating errors in measuring the length of segments defined between teeth of a speed signal transmitter wheel. The Mezger, et al., system provides such compensation by xe2x80x9clearningxe2x80x9d the segment lengths within a predetermined engine speed range, adaptively determining correction values therefor, and applying such correction values for all subsequently measured engine speeds. However, the xe2x80x9clearningxe2x80x9d process is elaborate, involving multiple speed ranges. Furthermore, the system involves a device in addition to a normal engine system. U.S. Pat. No. 5,345,817 to Grenn, et al., discloses a speed signal correction system wherein the effects of tooth-to-tooth spacing errors are avoided by using common points on the crank tone wheel rather than comparing neighboring points on the wheel. Grenn""s system is designed for the detection of strong engine disturbances which affect engine speed for more than one cylinder cycle; minor fluctuations within one engine cycle are filtered out.
Therefore, there is a need for a method and a system which are straightforward and inexpensive for removing static errors from engine speed data, such that the resultant engine speed data better reflect the true speed of the engine. In addition, this method or system has to be applicable over the operation speed range of the engine.
This invention calculates a least squares regression of engine speed data measured at specific target distances when the engine is in motoring deceleration without combustion or load. Recognizing that the average cylinder speed should form a straight line within an engine cycle, the least squares regression of the measured speed data therefore represents the theoretical performance of the engine. Deviations from this theoretical performance are grouped together as static errors. A residual or delta between the average cylinder speed (represented by the linear regression line) and the actual measured engine speed can therefore be determined. This delta error versus crank angle can later be subtracted from the engine speed measurements to remove the static errors.
In one form of the invention, a method for correcting static errors in measured engine speed is disclosed, comprising the steps of: a) sensing a speed of said engine a plurality of times during a time period; b) performing a least square regression analysis upon said sensed engine speeds, thereby determining a theoretical engine speed line; c) subtracting said engine speed from said theoretical speed to determine a residual at each speed measuring point; d) measuring an instantaneous engine speed; and, e) subtracting said residual from said measured engine speed, thereby resulting in a compensated engine speed which is a closer estimate of the theoretical speed of said engine.
In another form of the invention, a method for compensating measured engine speeds for non-fuel induced imbalances in a multi-cylinder reciprocating internal combustion engine is disclosed, comprising: a) sensing signals as targets pass by for a time period; b) measuring a speed at a plurality of speed points, wherein said speed points each span a plurality of consecutive targets; c) grouping said speed points, wherein each group defines a pattern of targets in one crankshaft revolution; d) averaging said speed measured, for each said speed points, over all speed point groups; e) performing a least squares regression analysis upon said average measured engine speed, thereby determining a theoretical engine speed line; f) determining a residual at each speed measuring point; g) measuring an instantaneous engine speed; and, h) subtracting said residual from said instantaneous engine speed, thereby resulting in a compensated engine speed which is a closer estimate of a true speed of said engine.
In yet another form of the invention, a system for compensating measured engine speeds for non-fuel induced imbalances in a multi-cylinder reciprocating internal combustion engine is disclosed, comprising: means for sensing a speed of said engine a plurality of times during a time period; means for performing a least squares regression analysis upon said sensed engine speeds, thereby determining a theoretical engine speed line; means for determining a residual at each speed measuring point; means for measuring an instantaneous engine speed; and means for subtracting said residual from said measured instantaneous engine speed, thereby resulting in a compensated engine speed which is a closer estimate of said theoretical speed of said engine.
In a further form of the invention, a system for compensating measured engine speeds for static errors is disclosed, comprising: means for sensing a target during a rotation of the crankshaft; means for computing a measured engine speeds at a plurality of speed points; means for performing a linear regression upon said measured engine speeds, thereby producing a mathematical representation of an average speed of said engine regardless of the firing of said at least one cylinder; means for computing a residual; means to measure an instantaneous engine speed; and means to calculate compensated measured speeds using said residuals, such that said compensated measured speeds better approximate a true speed of said engine.
A principal object of the present invention is to provide a software method and system to compensate for non-fuel injection induced errors (static errors), such as those caused by mechanical parts and installation tolerances, in measured engine speed data.
Another object of the present invention is to provide a look-up table which is accessible to the engine control microprocessors (ECM) of the engine to compensate for static errors in engine speed data measured over the operating speed range of the engine.
These and other objects and advantages of the invention herein will be set forth in the following description made in connection with the accompanying drawings in which like reference characters refer to similar parts throughout the several views.